Rhabdomyolysis is a condition where muscle cells explode after extremely strenuous activity, releasing myoglobin into the bloodstream. Myoglobin is toxic to your kidneys and can result in kidney failure and/or death. If you survive, there’s a chance you could suffer permanent muscle damage and strength loss.

While rhabdomyolysis is considered to be somewhat rare, Robertson says CrossFit trainers seem to be very familiar with the disorder. Apparently, “rhabdo” is quite common amongst CrossFit circles, and has even inspired internal documentation and a cartoon character, “Uncle Rhabdo”.

One CrossFit document, authored by Will Wright, M.D., suggests the best way to attack rhabdomyolysis is to prevent it from happening. Wright proposes trainers and coaches should take the time to vet new and potential members about their conditions, fitness levels, etc. and then introduce them to CrossFit at a gradual pace.

Of course, a “gradual” pace may not be so in line with CrossFit’s fast and extreme workout philosophy. In a New York Times article, an Army Ranger went right back into the workout of the day or WOD after suffering from rhabdomyolysis just six months prior. “I see pushing my body to the point where the muscles destroy themselves as a huge benefit of CrossFit,” he said.

So CrossFit now has two dirty little secrets: too much of it can seriously maim or kill you and kip pull-ups aren’t really pull-ups (sorry, guys).